6.4: Phosphorus Cycling and Flows

Phosphorus, like nitrogen, is a macronutrient anion. And like N it also cycles in the soil (though not in as complicated a way as nitrogen).

Figure \(\PageIndex{1}\): The 'natural' Phosphorus Cycle: Reworked by Bonniemf from the public domain file. CC - Wikimedia

Figure \(\PageIndex{1}\) shows that rock weathering from the earth's crust had been the only way geological phosphorus came into the biological realm. For this reason, P has been described as a bottleneck element--a plant or a field is often limited by how much of this nutrient it can lay its roots on. As Julia Rosen writes in her 2021 article in the Atlantic magazine, "Organisms have generally had to wait for geologic forces to crush, dissolve, or otherwise abuse the planet’s surface until it weeps phosphorus. This process of weathering can take thousands, even millions, of years. And once phosphorus finally enters the ocean or the soil, where organisms might make use of it, a large fraction reacts into inaccessible chemical forms."

Phosphate ions (H2PO4-) are the forms the plant takes up from soil/water solution and immobilizes in its body.

Phosphorus does not enter into a gaseous state and causes greenhouse gas problems like Nitrogen. It does not often leach through the soil either unless it is quite excessive. So the P cycle is simpler than the N cycle because it does not leak P via gas or leaching.

Phosphorus does, however, cause serious environmental problems because the H2PO4- ion is not stable in soil solution like the nitrate ion. It adsorbs onto clay particles. When the clay topsoil is washed into streams and rivers via rainfall, the phosphate ion goes with it. Then, it “releases” when it is in the water. Algae blooms are fueled by too much phosphorus entering the system. The resultant explosion of algae sucks the oxygen out of the water causing fish and other aquatic organisms to die, the water body becomes hypoxic. The Bay of Green Bay (Wisconsin) has 'always' been green because of the abundant phosphorus released from the clay soils in the watershed. One local stream was named 'glaice' or clay creek by the first French traders to make the area their home.

What the first diagram of the phosphorus cycle does not show, but the diagram below does, is the influence of human activity on the cycle.

Figure \(\PageIndex{2}\): The Phosphorus Cycle with human interference. Author: Valerie Dantoin 2025

What happens to all the phosphorus that accumulates in cities in the form of human bodies and human excrement? That is where there is a break in the cycle. The phosphorus is not returning to the soil, it is sent into the ground in human bones, but they are locked far away from farm fields. Phosphorus is not removed during sewage treatment and it is discharged into waterways of cities and towns. It flows out into large water bodies and settles at the bottom. In millions of years, as the earth changes and seas rise and fall, the phosphorus may again become available as dried sediment if there are any humans around to mine it. Because there is a finite amount of phosphorus and because we humans are sealing it away in our bones or peeing it into our waterways where it becomes a pollutant - we remove it from the farm cropping system and it will run out.

The Natural P Cycle and Human Activity - Julia Rosen

"Over the course of several weeks in 1669, a German alchemist named Hennig Brand boiled away 1,500 gallons of urine in hopes of finding the mythical philosopher’s stone. Instead, he ended up with a glowing white substance that he called phosphorus, meaning “light bearer.” It became the 15th element in the periodic table, the incendiary material in matches and bombs, and—thanks to the work of Liebig and others—a key element in fertilizer.

Long before phosphorus was discovered, however, humans had invented clever ways of managing their local supplies, says Dana Cordell, who leads the food-systems research group at the University of Technology Sydney, in Australia. There and in the Americas, for example, Indigenous people managed hunting and foraging grounds with fire, which effectively fertilized the landscape with the biologically available phosphorus in ash, among other benefits. In agrarian societies, farmers learned to use compost and manure to maintain the fertility of their fields. Even domestic pigeons played an important role in biblical times; their poop—containing nutrients foraged far and wide—helped sustain the orchards and gardens of desert cities.

By the 1800s, centuries of cultivation had sapped Britain’s soils of nutrients, including phosphorus—an essential element for crops. At the time, manure and bones were common sources of phosphorus, and when the country exhausted its domestic reserves, it looked elsewhere for more.

“Great Britain is like a ghoul, searching the continents,” wrote Justus von Liebig, the German chemist who first identified the critical role of phosphorus in agriculture. “Already in her eagerness for bones, she has turned up the battlefields of Leipzig, of Waterloo, and of the Crimea; already from the catacombs of Sicily she has carried away the skeletons of many successive generations.”

Then, in the 1840s, geologists discovered phosphorus-rich stones buried in the fields around Cambridge (England)" ...

"Over the ensuing decades, workers extracted 2 million tons of coprolites, transforming the fields and fens of southeast England into a warren of pits and trenches that dwarfed Simon’s hole. Coprolites were sorted, washed, and transported by buggy, train, and canal barge to processing facilities, where they were milled and treated with acid to make superphosphate—the world’s first chemical fertilizer.

The rocks helped Britain boost its food supply and consummate the so-called Second Agricultural Revolution (the first “revolution” being the rise of agrarian civilization). Coprolites and other geologic deposits of phosphorus also raised the tantalizing possibility that humans had at last broken free of an age-old biological constraint. For billions of years, life on Earth had struggled against a stubborn lack of phosphorus. Finally, that was about to change."

Seabird guano (manure) from islands in the Pacific Ocean was mined by workers (and some slave labor) in the late 1800s. This mining industry sent phosphorus by the ton, in sailing ships to Europe to fertilize fields and uncork the phosphorus bottleneck. Bat guano from caves was also mined by imperial industries until it was all gone. Of course, mining operations disrupted the birds and the bats so no new deposits accrued.

Figure \(\PageIndex{3}\): Mining guano in the South Pacific. CC Public Domain [creativecommons.org]

In the United States, bones of bison slaughtered on the Great Plains were collected and ground for phosphorus fertilizer. In the late 1800s phosphate mines began operations near Charleston South Carolina--digging the rock sediments of long-dead sea life and ocean deposits. Most P now comes from sediments in Florida where problems with mining and evaporation ponds cause their own environmental issues.

Again, Rosen writes: "These deposits became increasingly important in the 20th century, during the Green Revolution (the third revolution in agriculture if you’re keeping track). Plant breeders developed more productive crops to feed the world and farmers nourished them with nitrogen fertilizer, which became readily available after scientists discovered a way of making it from the nitrogen in the air (Haber Bosch). Now, the main limit to crop growth was phosphorus—and as long as the phosphate mines hummed, that was no limit at all. Between 1950 and 2000, global phosphate rock production increased sixfold and helped the human population more than double.

But for as long as scientists have understood the importance of phosphorus, people have worried about running out of it. These fears sparked the fertilizer races of the 19th century as well as a series of anxious reports in the 20th century, including one as early as 1939, after President Franklin D. Roosevelt asked Congress to assess the country’s phosphate resources so that “continuous and adequate supplies be insured.”

Figure \(\PageIndex{4}\): A phosphate mine in Morrocco. Kopalnia piasku Dargocice. Web Archive [web.archive.org] CC BY

If phosphorus is a limiting factor in plant growth, a weak link in the chain, that means we do not have unlimited capacity to continue growing the human population. A 'new' source of phosphorus must be found and perhaps that source is recycling. Not recycling of metal cans or cardboard or plastics.... but of poop and pee.

Julia Rosen writes "... human waste was perhaps the most prized fertilizer of all. Though we too need phosphorus (it accounts for about 1 percent of our body mass), most of the phosphorus we eat passes through us untouched. Depending on diet, about two-thirds of it winds up in urine and the rest in feces. For millennia, people collected these precious substances—often in the wee hours, giving rise to the term night soil—and used them to grow food."

There are many examples of civilizations that realized the importance of recycling their 'waste' and using it to continue producing food. Rosen continues "The sewage of the Aztec empire fed its famous floating gardens. Excreta became so valuable that authorities in 17th-century Edo, Japan, outlawed toilets that emptied into waterways. And in China the industry of collecting night soil became known as “the business of the golden juice.” In Shanghai in 1908, a visiting American soil scientist named Franklin Hiram King reported that the “privilege” of gathering 78,000 tons of human by-products cost the equivalent of $31,000."

F.H. King was a soil scientist who wrote the book 'Farmers of Forty Centuries' explaining the way Chinese farmers on terraced hillsides kept their land productive by recycling human waste. The soils department building at the University of Wisconsin Madison is named after him. He understood that our modern way of taking care of our sewage using clean water for flushing, was not, perhaps the best way. It certainly failed to recycle the P in our pee or poop. King wrote that although 'sanitation' was a great achievement because it reduced some biological illnesses found in sewage where large numbers of humans congregate, it failed to recognize the importance of recapturing phosphorus (and nitrogen) in our own manure.

Of course, back in the 1900s it would have been difficult to collect, transport, and spread human waste back onto farm fields. But this break in the P cycle was exactly what early thinkers in economics would describe as a metabolic rift. The rift is a break in the cycle and the change in flows from sources to new sinks. As Rosen pictures it, the loop has become a one-way pipe. It can also be pictured as a map of the U.S. where the Pacific coast and the Atlantic coast are made of mountain ranges, and the ranges are piles of human bones and excrement. On the map, in the middle of the country is a deep valley where the nutrients have been mined and transported to the human population mountains on the coasts. Its an unsettling image, but one that depicts how a source (the American Midwest) is hollowed out to build up the coastal mountains - the final resting place of the continent's phosphorus and other nutrients.

She continues “That single disruption has caused global chaos, you could argue,” Cordell says. For one thing, it forced farmers to find new sources of phosphorus to replace the nutrients lost every year to city sewers. To make matters worse, agricultural research in the late 1800s suggested that plants required even more phosphorus than previously thought. And so began a frantic race for fertilizer. These events raised a terrifying possibility: What if the phosphorus floodgates were to suddenly slam shut, relegating humanity once more to the confines of their parochial phosphorus loops? What if our liberation from the geologic phosphorus cycle is only temporary?

Dan Eagan writes in The Devils Element that 'The potential benefits to better managing manure are staggering, both in protecting water quality and preserving phosphorus rock reserves for future generations'. He is writing about cow manure, but the same applies to human manure. Eagan quotes Jim Elser director of the Sustainable Phosphorus Alliance, "If all manures were recycled and returned for [agricultural] production, I think you could displace half of the mined fertilizer."

We have a strong prohibition against human waste in the U.S. because of potential pathogens. We seem to treat it like it's poisonous. We have crafted laws that regulate any manure, let alone human manure, from touching or contacting our vegetables and fruits. However, these regulations do take into account that if a raw vegetable is cooked (has a 'kill' step) then it can come in contact with manure used as fertilizer. There are numerous ways to limit pathogens in produce. It is safe to use manures, for example, if they go through a composting process or if there is a waiting period between application and vegetable harvest. We as a society just have to get over the 'ick' factor. Some scientists are experimenting with new toilets that conserve water (up to 7 gallons for every flush according to Eagan) and also collect and concentrate the nutrients. There are companies working on systems that retrieve and concentrate P from municipal sewage plants. Some European countries have passed laws that require P recovery from sewage.

The U.S. used to be an exporter of P but has now become a net importer. Estimates vary on how much P there is left to mine in deposits around the world. The Moroccan deposits in the Western Sahara desert as well as deposits in China may only have a few decades left before they are scraped dry, just like the once mountainous reserves on the Guano islands in the Pacific. Of course, both countries' mines are subject to political instability as well. Other experts in the industry claim there are 350 years worth of P left in global deposits and that we will find more ways to find more supply as we have done with fossil fuels. Perhaps rocks on the floor of the ocean? It will of course take more energy to extract these remote P resources and then we still have the problem that we re-deposit P in our skeletons and excrete P fertilizer inputs out into our waterways via pee.

Eagan notes that it is a bit ironic that it has been about 350 years since Hennig Brandt discovered phosphorus and now we are facing a potential reduction in crop yields as P runs out. However, it is exciting to consider that maybe we are beginning to realize we have to 'do agriculture' and take care of nutrients in the soil in more careful ways. Maybe we are at the bottom of a pit where we have extracted resources and spilled them onto the landscape and into the water without care. Maybe we will understand that to farm sustainably we have to recover and reuse our precious phosphate if we are to dig our way out from the bottom and reach sustainability.